1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.
2. Related Art
In a silicon-germanium (xe2x80x9cSiGexe2x80x9d) heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The SiGe HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Cutoff frequencies in excess of 100 GHz, which are comparable to the more expensive gallium-arsenide based devices, have been achieved for the SiGe HBT.
The higher gain, speed and frequency response of the SiGe HBT are possible due to certain advantages of silicon-germanium, such as a narrower band gap and reduced resistivity. These advantages make silicon-germaniun devices more competitive than silicon-only devices in areas of technology where high speed and high frequency response are required.
The advantages of high speed and high frequency response discussed above require the realization of a thin highly doped base layer in the NPN SiGe HBT. For example, boron is commonly utilized to provide P-type doping of the base in an NPN silicon-germanium HBT. However, boron has a tendency to diffuse in the base. In other words, the boron profile in the base has a tendency to widen, thus undesirably widening the base. Boron diffusion is further accelerated during subsequent thermal processing steps that occur in the fabrication of the NPN SiGe HBT. The increased boron diffusion can severely degrade the high frequency performance of the NPN SiGe HBT. Thus, suppression of boron diffusion presents a major challenge in the fabrication of a NPN SiGe HBT.
One method of suppressing boron diffusion in the base of the NPN SiGe HBT is by adding carbon in the base. For example, a concentration of greater than 1*1019 of carbon atoms per cubic centimeter can be added in the base of the NPN SiGe HBT at the point where the concentration of boron peaks. Although adding carbon in the base effectively suppresses boron diffusion, the addition of carbon has the undesirable effect of causing a band gap discontinuity at the collector-base junction. As a result of the band gap discontinuity at the collector-base junction, the electrical performance of the NPN SiGe HBT is accordingly diminished. For example, the above band gap discontinuity can increase the base transit time of electrons moving from the emitter to the base, thereby limiting the cut-off frequency of the NPN SiGe HBT.
Graph 100 in FIG. 1 shows conventional exemplary boron, carbon, and germanium profiles in a base in an NPN SiGe HBT. Graph 100 includes concentration level axis 102 plotted against depth axis 104. Concentration level axis 102 shows relative concentration levels of boron, carbon and germanium. Depth axis 104 shows increasing depth into the base, starting at the top surface of the base, i.e. at the transition from emitter to base in the NPN SiGe HBT. The top surface of the base in the NPN SiGe HBT corresponds to xe2x80x9c0xe2x80x9d on depth axis 104. The bottom surface of the base, i.e. the collector-base junction, corresponds to depth 122 on depth axis 104.
Graph 100 also includes boron profile 106, which shows the concentration of boron in the base, plotted against depth, i.e. distance into the base. Boron profile 106 includes peak boron concentration level 108, which occurs at depth 114. Graph 100 further includes carbon profile 112, which shows the concentration of carbon in the base, plotted against depth. The concentration of carbon in carbon profile 112 increases abruptly from 0.0 to a constant level at depth 114, and remains at a constant level from depth 114 to depth 122. At depth 122, the carbon concentration level decreases abruptly to 0.0.
Graph 100 further includes germanium profile 116, which shows the concentration of germanium in the base of the present exemplary NPN SiGe HBT, plotted against depth. Germanium profile 116 begins at 0.0 concentration level at depth 110 and increases to depth 118. Germanium profile 116 maintains a constant concentration level from depth 118 to depth 120. At depth 120, germanium profile 116 decreases to 0.0 concentration level at depth 122. Thus, a concentration of carbon is added in the base of the NPN SiGe HBT at depth 114, which corresponds to peak boron concentration level 108.
Graph 200 in FIG. 2 shows a conventional exemplary band gap curve in the base and at the collector-base junction in the conventional exemplary NPN SiGe HBT. Graph 200 shows band gap curve 202, which shows the change in band gap caused by carbon profile 112 and germanium profile 116 in FIG. 1 in the base in the present exemplary NPN SiGe HBT. Graph 200 includes change in band gap axis 208 plotted against depth axis 204. It is noted that xe2x80x9c0xe2x80x9d on change in band gap axis 208 refers to the band gap of a reference base comprising only silicon, i.e. a silicon-only base. It is also noted that an upward move on band gap curve 202 indicates a decrease in the band gap of the present exemplary NPN SiGe HBT relative to the band gap of a silicon-only base. Conversely, a downward move on band gap curve 202 indicates an increase in the band gap relative to the band gap of a silicon-only base.
Depth axis 204 corresponds to depth axis 104 in FIG. 1. In particular, depths 210, 214, 218, 220, and 222, respectively, correspond to depths 110, 114, 118, 120, and 122 in FIG. 1. At depth 210, band gap curve 202 begins to decrease. As is known in the art, an increase in the concentration of germanium in a base of an NPN SiGe HBT results in a decrease in band gap. Thus, band gap curve 202 decreases from depth 210 to just prior to depth 214 as the result of a ramp up in concentration of germanium. At depth 214, band gap curve 202 indicates an abrupt increase in band gap. This step increase in band gap corresponds to the addition of carbon in the base at depth 114 in FIG. 1.
Band gap curve 202 decreases from depth 214 to depth 218 as the result of a ramp up in concentration of germanium. Between depth 218 and depth 220, band gap curve 202 remains constant as a result of a constant concentration of germanium. Between depth 220 and depth 223, band gap curve 202 increases as a result of a ramp down in concentration of germanium. At depth 223, band gap curve 202 continues to increase to band gap level 224 as a result of a constant concentration of carbon and the ramp down in concentration of germanium.
At approximately depth 222, i.e. at the approximate collector-base junction of exemplary NPN SiGe HBT, band gap curve 202 abruptly decreases to the reference band gap of a silicon-only base. Distance 226 refers to the distance between depth 223, i.e. the depth at which band gap curve 202 crosses depth axis 204, and approximately depth 222, i.e. the approximate depth where band gap curve 202 abruptly decreases to the reference band gap of a silicon-only base. For example, distance 226 can be approximately 50.0 to 100.0. The band gap discontinuity, i.e. the abrupt decrease in band gap, at approximately depth 222 is caused by the abrupt decrease in the concentration level of carbon at depth 222. As such, the rapid decrease in carbon at the collector-base junction of conventional exemplary NPN SiGe HBT results in an undesirable band gap discontinuity at the collector-base junction.
Thus, there is a need in the art to provide a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing an undesirable band gap discontinuity at the collector-base junction.
The present invention is directed to method and structure for eliminating collector-base band gap discontinuity in an HBT. The present invention overcomes the need in the art for a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing an undesirable band gap discontinuity at the collector-base junction.
According to one exemplary embodiment, a heterojunction bipolar transistor comprises a base having a concentration of germanium, where the concentration of germanium decreases between a first depth and a second depth in the base. For example, the concentration of germanium may decrease at a linear rate between the first depth and the second depth.
According to this exemplary embodiment, the base of the heterojunction bipolar transistor further comprises a concentration of a diffusion suppressant of a base dopant, where the concentration of the diffusion suppressant decreases between a third depth and a fourth depth so as to counteract a change in band gap in the base between the first depth and the second depth. For example, the diffusion suppressant can be carbon and the base dopant can be boron. The concentration of the diffusion suppressant, for example, may decrease at a linear rate between the third depth and the fourth depth, and the third depth may be situated in a germanium plateau region in the base. For example, the concentration of the diffusion suppressant may decrease between the third depth and the fourth depth so as to counteract the change in band gap in the base at approximately the second depth.